Tuesday, August 07, 2012

The Basics of Genetic Genealogy

"THE BASICS OF GENETIC GENEALOGY," by Guido Deboeck

In spite of the numerous sources and vast amount of documented information that can be found, we should not forget that documents can contain errors, may have been written to mislead or hide the truth, or may have been destroyed either on purpose or by accident (e.g. by fire, floods, or earthquakes). Additionally, some relationships may never have been recorded. This is why conventional genealogy can only go as far as the research of documents allows.To go beyond the constraints of the paper world, there is genetic genealogy. Genetic genealogy relies on DNA, which we all have, does not change, cannot be destroyed, and is never wrong.

DNA testing complements conventional genealogy through the analysis of the unique sequence of chemicals that defines each human being. Through DNA testing, one can tell if two people are related (though not the exact nature of the relationship), verify, or potentially correct genealogical information extracted from documents.

DNA contains the blueprint of life, i.e., all the instructions that build and control the day-to-day functioning of the cells in our body. This blueprint, with its instructions, is passed from parent to child with few or minor changes.

DNA stands for DeoxyriboNucleic Acid. It is structured as a double-stranded helical molecule. Think about a ladder with rungs or sides that are twisted: the rungs are composed of chemicals held together with a sugar backbone, somewhat like table sugar. These chemicals are called nucleic acid bases, or nucleotides; they are the building blocks of every DNA molecule. The four nucleotides contained in every DNA molecule are Adenine, Cytosine, Guanine, and Thymine, which are simply labeled as A, C, G, and T.

A section of the long, double-stranded helical DNA molecule is a gene. A gene contains instructions for some specific functions, such as making a protein. Some genes are responsible for physical characteristics.

There are about 30,000 genes, but they constitute less than five percent of all DNA. The rest are commonly called "junk DNA," although some parts of this DNA determine the structure of the chromosomes. Genes are packaged in 46 chromosomes, which are arranged in 23 pairs that define the human genome. In sum, the complete human genome contains billions of bits of information.

Children inherit copies of their parents' DNA. This genetic hand-off is repeated from generation to generation. In copying DNA, some mistakes may on occasion occur, for example, the substitution of a C for G or a T for an A. Think about monks in a medieval monastery who copied manuscripts and on occasion made some mistakes (maybe after finishing their daily ration of five liters of beer). Despite the efforts of even the abbot of the monastery, who proofread all pages that the monks copied, spelling mistakes may have remained in the final document.

The same happens when proofreading a book: several mistakes may be removed but despite my best efforts some mistakes still remain. In genetics, such mistakes are called "mutations." These mutations provide variation, or the evolution of the basic building blocks. However, mutations occur at a low rate, maybe 50 changes per generation in billions of nucleotides that make up the human genome.

How can this evolution in the building blocks be useful to genealogists? The DNA in the nucleus of a cell contains 23 pairs of chromosomes. One pair determines the gender. Males receive or inherit a Y chromosome from their father and an X chromosome from their mother. Females inherit two Xs. Hence, males with the same Y chromosome have a common ancestor. Y chromosome analysis (Y-DNA) can verify or help to investigate the paternal lineage of an individual. Investigation of the X chromosomes in a female can verify her paternal lineage only if the X that is common between two sisters is the same as the X chromosome of their father.

Outside the nucleus of a cell are many small organelles, called "mitochondria." These are the power stations of a cell because they are structures in which energy is produced and stored. Mitochondrial DNA (mtDNA) is the small amount of DNA found in the mitochondrion. mtDNA is passed on via the egg cell of a mother, hence only females can pass mtDNA to their offspring. In consequence, an analysis of the mtDNA in males or females can provide valuable information to verify or help to investigate the maternal lineage of an individual. In this way, genetic genealogy can identify paternal lineage via Y chromosome analysis and identify the maternal lineage via mtDNA analysis. By testing males for both Y-chromosome and mtDNA, one can trace their paternal and maternal lines. By testing females for mtDNA, one can trace their maternal lines. The information obtained through these analyses can determine the specific branches via which an individual comes from the evolutionary tree of human relationships. DNA testing complements conventional genealogy. Both conventional and genetic genealogy can contribute to a more comprehensive family history.

About the author:Guido Deboeck, Ph.D., is the author of FLEMISH DNA & ANCESTRY: History of Three Families over Five Centuries Using Conventional and Genetic Genealogy. This book is a model case study in the application of DNA research in genealogy as well as a thorough genealogy of some prominent Flemish families.

1 comment:

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